Process for selective disproportionation of toluene and disproportionation and transalkylation of toluene and C9+ aromatics

ABSTRACT

The present invention relates to a process for the selective disproportionation of toluene and the disproportionation and transalkylation of toluene and C 9   +  aromatics to mainly solve the problems in the prior arts of the great amount of recycle stream, high energy consumption or harsh requirement for the reaction feedstocks. The present invention has better solved these problems by the technical solution using a process for selective disproportionation of toluene to produce mixed xylenes containing a high concentration of p-xylene, and subsequent disproportionation and transalkylation of C 9   +  aromatics and toluene to produce benzene and the mixed xylenes which are in the thermodynamic equilibrium. The process is applicable to the industrial production.

PRIORITY CLAIM

This application claims priority from China Patent Application Number01131953.4 which was filed on Oct. 22, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the selectivedisproportionation of toluene, and the disproportionation andtransalkylation of toluene and C₉ ⁺ aromatics (C₉ ⁺A), in particular itrelates to a process for the selective disproportionation of toluene,and the disproportionation and transalkylation of toluene and C₉ ⁺aromatics followed by a process for producing p-xylene (PX) by atechnique of the isomerization of C₈ aromatics (C₈A) and a technique ofp-xylene separation.

2. Description of the Related Art

p-xylene is one of the major basic organic feedstocks in petrochemicalindustry and has widespread applications in many fields such as chemicalfiber, synthetic resin, pesticide, medicine, plastic, etc. Thetraditional process for producing p-xylene is shown in FIG. 1. C₈aromatics (C₈A) in thermodynamic equilibrium produced from the catalyticreforming process of naphtha passes through a multi-stage subzerocrystallization separation or molecular sieve simulation moving bedseparation (abbreviated as adsorptive separation) to separate p-xylenefrom its isomer mixture with near boiling points. A C₈A isomerization(abbreviated as isomerization) process is generally used to isomerizeo-xylene and m-xylene to p-xylene. The use of disproportionation oftoluene, or the disproportionation and transalkylation of toluene and C₉⁺ aromatics (C₉ ⁺A) to produce benzene and C₈A, is an effective routefor increasing the output of p-xylene.

So far, the typical and mature processes relating to toluenedisproportionation in the world are the Tatoray traditional toluenedisproportionation process of UOP Inc. industrialized in the end of1960s, the MTDP process of Mobil Chemical Company developed in the endof 1980s, the S-TDT process of Shanghai Institute of PetrochemicalIndustry developed recently, and the TransPlus transalkylation processof heavy aromatics of Mobil Chemical Company. The selectivedisproportionation of toluene is a new route for producing p-xylene.Since the selective disproportionation of toluene on modified ZSM-5catalysts can produce benzene and C₈A with a high concentration ofp-xylene, the difficulty in the separation of p-xylene is greatlyalleviated. In recent years, along with the improvement of the catalystperformance, this process has made a great progress. The typicalprocesses are the Mobil toluene selective disproportionation processMSTDP industrialized in late 1980s and the UOP toluene selectivedisproportionation process PX-Plus developed in late 1990s.

In the industrialized toluene selective disproportionation processMSTDP, a converted ZSM-5 mesoporous molecular sieve is used as thecatalyst to treat a toluene feedstock yielding C₈A with a highconcentration of p-xylene (85-90% by weight, the same below exceptotherwise noted) and nitration grade benzene. In the PX-Plus process, noindustrialization of which has been reported, to applicant's knowledge,the major reaction performance is a PX selectivity of 90%, a mole ratioof benzene to PX of 1.37 in case of the toluene conversion of 30%.

Nevertheless, in these toluene selective disproportionation processes, ahigh p-xylene selectivity is accompanied by a harsh requirement to thefeedstock selection, and only toluene can be used as the feedstock. C₉⁺A has no use for these processes, and at least it can not be directlyused. This is a vital shortcoming of the selective disproportionationprocess.

The feed for the reactor of a typical Tatoray process is toluene and C₉aromatics (C₉ ⁺A). The xylenes produced in the Tatoray process are amixture of the isomers in thermodynamic equilibrium, and generally, thecontent of p-xylene having the most industrial application value is onlyabout 24%. Compared to the selective disproportionation of toluenewherein mixed xylenes having a p-xylene concentration of about 90% canbe obtained, the Tatoray process is obviously inferior in this aspect,but a great advantage of the Tatoray process over the toluene selectivedisproportionation process is that the Tatoray process can convert C₉Ato benzene and xylenes.

The literature based on the Tatoray Process includes U.S. Pat. Nos.4,341,914, 2,795,629, 3,551,510, CN98110859.8, CN97106719.8, etc. Theprocess disclosed by the representative U.S. Pat. No. 4,343,914comprises conducting the aromatics fractionation of the reformedproduct, feeding the derived toluene and C₉A to the Tatoray unit toconduct disproportionation and transalkylation reactions, recyclingtoluene, C₉A, and a part of C₁₀ aromatics (C₁₀A) after separating theproducts, withdrawing benzene as a product, feeding xylene together withthe xylene from the isomerization unit to the PX separation unit toseparate the high purity of p-xylene product. The other isomers ofxylene are fed to the isomerization unit to conduct the isomerizationreaction of xylene, yielding the mixed xylenes in thermodynamicequilibrium again.

One representative aromatics complex based on the selectivedisproportionation process is the PX-Plus Process. The greatestdifference between the PX-Plus Process and the traditional aromaticscomplex Tatoray process is that C₉ ⁺A is withdrawn as a byproductinstead of as a feed in the PX-Plus process.

It is readily seen from summarizing the above processes that thetraditional aromatics production process uses the disproportionation andtransalkylation process to attain the object of increasing the output ofxylenes, but the amount of the recycled xylenes is large since theproduced mixed xylenes are the isomers of xylene in the thermodynamicequilibrium and therefore the concentration of p-xylene is low. Theother isomers of xylene have to pass through the isomerization unit tobe converted to p-xylene, thus resulting in heavy xylene recycles andhigh energy consumption in the isomerization unit, PX separation unitand aromatics fractionation unit. Although mixed xylenes with highconcentration of p-xylene can be obtained by the aromatics productionprocess using the selective disproportionation process, and thereby thefeeds of the isomerization unit and the like are greatly reduced, theselective disproportionation process can not treat C₉ ⁺A, which resultsin a waste of the C₉ ⁺A resource, and a reduced output of the targetproduct, p-xylene.

Therefore, it is very desirable to develop a process for producing highyield of the desire product p-xylene, which does not have harsh strictrequirement to the feedstock. It is also desirable to greatly reduce theenergy consumption or increase the capacity of processing the feedstock.

SUMMARY OF THE INVENTION

Thus, one object of the present invention is to overcome theshortcomings present in the prior art of low concentration in the mixedxylenes, high energy consumption, or harsh requirement to the reactionfeedstock in the production of p-xylene.

Another object of the present invention is to provide a novel processfor the selective disproportionation of toluene and thedisproportionation and transalkylation of toluene and C₉ ⁺ aromatics.The improved economical viability and profitability of producingp-xylene are obtained by increasing the capacity of the whole processand the output of the target products p-xylene and benzene, greatlyreducing the scale of the p-xylene separation unit, isomerization unit,and the aromatics fractionation unit, and thereby decreasing the energyconsumption of the whole process.

These and other objects are attained by a novel process of the selectivedisproportionation of toluene and the disproportionation andtransalkylation of toluene and C₉ ⁺ aromatics comprising:

a) separating a feed stream comprising benzene, toluene, C₈ aromatics,and C₉ ⁺ aromatics into a first benzene stream, a toluene stream, afirst C₈ aromatics stream, and a C₉ ⁺ aromatics stream;

b) introducing a part of the toluene stream into a toluene selectivedisproportionation unit to conduct the toluene selectivedisproportionation reaction for producing a first effluent comprising C₈aromatics rich in p-xylene, and benzene;

c) separating a second C₈ aromatics stream and a second benzene streamfrom the first effluent produced in the step of b);

d) introducing another part of the toluene stream and the C₉ ⁺ aromaticsstream into a toluene disproportination and transalkylation unit toconduct the toluene disproportionation and transalkylation reaction inthe presence of hydrogen for producing a second effluent comprising C₈aromatics and benzene;

e) separating a third C₈ aromatics stream and a third benzene streamfrom the second effluent produced in the step of d); and

f) separating the first, and third C₈ aromatics stream into a p-xyleneproduct and a remaining mixed xylenes stream.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrateprocedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is the flow diagram of the traditional complex for aromaticsproduction.

FIG. 2 is the flow diagram of the process for selectivedisproportionation and disproportionation and transalkylation ofxylene+C₉ ⁺A for producing p-xylene in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Typically, the feed stream comprising benzene, toluene, C₈ aromatics,and C₉ ⁺ aromatics may be from a catalytic reforming unit where thefeedstock naphtha is converted to an oil rich in aromatics which furtherundergoes the treatment of depentanizing and aromatics extraction etc.The benzene separated from this feed stream may be sent out theseparation unit as a product. C₁₀ ⁺ aromatics, if present in the feedstream, may also be separated from this feed stream and sent out theseparation unit as a product, and the purity of C₉ aromatics in theseparated C₉ ⁺ aromatics stream may be further increased in this case.

In accordance with the present invention, the second C₈ aromatics has ahigh purity of the target product p-xylene, thus the second C₈ aromaticsrich in p-xylene may or may not be further separated to extract thep-xylene product. It is preferable to send the second C₈ aromatics to ap-xylene separation unit. The separation of the p-xylene target productfrom the second aromatics stream may be conducted independently or witheither or both of the first and the third aromatics stream together.Similarly, the separation of the p-xylene target product from the thirdaromatics stream may be conducted independently or with either or bothof the first and second aromatics stream together. It is preferable thatthe separation of the p-xylene from the first, the second, and third C₈aromatics stream is conducted in the same separation unit. The methodsof separating the p-xylene product from the C₈ aromatics stream includesadsorptive separation and subzero crystallization separation etc.

After the target p-xylene is separated, the remaining mixed xylene ispreferably sent to an isomerization unit to conduct the reaction ofisomerization for converting the remaining mixed xylenes to thep-xylene. The effluent of the isomerization unit may be sent to adeheptanizer where a C₈ ⁺A stream is withdrawn from the bottom, a streamcontaining benzene and toluene are withdrawn from the top. The C₈ ⁺Astream may be sent to an aromatics fractionation unit first, and the C₈aromatics separated may undergo a similar separation with the first,second, or the third C₈ aromatics stream as discussed above, so that thep-xylene product can be obtained.

The separation in the steps of a), c), and e) may be conductedindependently or with either or both of the other two steps together.For example, the first effluent and the second effluent may enter thesame aromatics fractionation unit as employed in the step of a).Similarly, the separation to C₈ ⁺A stream derived from the isomerizationunit may also be conducted independently or with at least one of thesteps of a), c), and e) together.

In accordance with one preferred embodiment of the present invention theseparation of the step of c) is specifically equipped with an aromaticsfractionation unit so that it can be conducted independently.Preferably, the second C₈ aromatics stream is further separated into apart of the p-xylene product and a residual liquid by the subzerocrystallization separation method. More preferably, the residual liquidmay be admixed with the first and the third C₈ aromatics stream to forma mixture, and then the mixture may be separated into the p-xyleneproduct and the remaining mixed xylene by the adsorptive separationmethod.

The normal operation conditions of the reaction zone of the tolueneselective disproportionation unit may be a pressure of 1-4 MPa, areaction temperature of 300-480° C., a hydrogen to hydrocarbon moleratio of 0.5-10, and a liquid weight space velocity of 0.8-8 h⁻¹; thecatalyst used in the toluene selective disproportionation unit may be aZSM-type molecular sieve catalyst, and preferably a ZSM-5-type molecularsieve catalyst; the SiO₂/Al₂O₃ mole ratio of the ZSM-5-type molecularsieve is 15-100; the ZSM-5-type molecular sieve preferably contains0.005-5%, preferably 0.01-1%, by weight of at least one metal or oxideof the element selected from the group consisting of platinum,molybdenum and magnesium. The normal operation conditions of thereaction zone of the disproportionation and transalkylation unit may beare a reaction pressure of 1-5 MPa, a reaction temperature of 250-480°C., a hydrogen to hydrocarbon mole ratio of 0.5-10, and a liquid weightspace velocity of 0.8-8 h⁻¹, the content of C₉ aromatics in the feed is5-60% by weight; the catalyst used in the disproportionation andtransalkylation unit is at least one molecular sieve selected from thegroup consisting of β-zeolite, mordenite, and MCM-22, with β-zeoliteor/and mordenite being preferred; the SiO₂/Al₂O₃ mole ratio of themolecular sieve is 10-100, preferably 10-50; the preferred catalyst is acatalyst containing metal bismuth or its oxide, the amount of which is0.005-5% by weight, preferably 0.05-2% by weight. The toluene from thearomatics fractionation unit is preferably the toluene separated in thearomatics fractionation unit or the toluene separated in the aromaticsfractionation unit and a foreign toluene from other available source asa supplement. The amount of the toluene entering into the tolueneselective disproportionation unit is preferably 5-95%, more preferably20-80% of the total amount of toluene. The weight ratio of the toluenefeed to the C₉ ⁺ aromatics in the disproportionation and transalkylationunit is 80/20-40/60.

In the toluene disproportionation and transalkylation unit of thepresent invention, the adopted bismuth-containing zeolite catalyst hasthe function to convert toluene and C₉ ⁺A to xylenes and benzene in thepresence of hydrogen through the disproportionation and transalkylationreactions. Because the decrease in the proportion of toluene andincrease in the proportion of C₉A in the feedstock benefit theoccurrence of the transalkylation reaction of toluene and C₉A⁺, more C₈Ais produced. In the toluene selective disproportionation unit, tolueneselective disproportionation reaction takes place, as a result, tolueneconverts to p-xylene with high selectivity. This kind of process routenot only can convert C₉ ⁺A to xylenes, eliminating the harsh requirementto the feedstock, but also can convert toluene to mixed xylenes with ahigh content of p-xylene through the selective disproportionation,further raising the concentration of the p-xylene in the mixed xylenefeed entering into the PX separation unit, facilitating the PXseparation, and lowering the scale of the isomerization unit and thelike, and thereby lowering the energy consumption of the whole aromaticscomplex and attaining a better technical effect. Besides, the presentinvention is especially applicable to the reconstruction of the existingaromatic complex for extension. The reconstruction is very simple:reserving the disproportionation and transalkylation, isomerization,p-xylene separation, and aromatics fractionation units as they are,setting up a new selective disproportionation unit. In this way, it ispossible to increase the output of p-xylene and benzene by raising thecapacity of the reforming unit or purchasing an adequate amount oftoluene, and meanwhile, the energy consumption is obviously lowered andbetter effects are obtained.

Now referring to FIG. 1 and FIG. 2, I represents the aromaticsfractionation unit, comprising a benzene separation tower, a tolueneseparation tower and a xylene separation tower, and optionally a heavyaromatics separation tower to be dependent on the concrete situation; IIrepresents the disproportionation and transalkylation unit; IIIrepresents the PX separation unit; IV represents the isomerization unit;and V represents the toluene selective disproportionation unit. Whetherunits II and V are equipped with the independent aromatics separationdevice of their own depends on the concrete situation. Line 1 is themixed stream of benzene and toluene, line 2 is the C₈ ⁺A streamwithdrawn from the bottom of the deheptanizer, line 3 is the high puritybenzene product withdrawn from the top of the benzene tower, line 4 isthe toluene withdrawn from the top of the toluene tower, line 5 is theC₈A withdrawn from the top of the xylene tower, line 6 is the C₉A and apart of C₁₀A withdrawn from the top of the heavy aromatics tower of thearomatics fractionation unit I, line 7 is C₁₀ ⁺A withdrawn from thebottom of the heavy aromatics tower, line 8 is the effluent withdrawnfrom the bottom of the stabilization tower of the disproportionation andtransalkylation unit, line 9 is the benzene product, line 10 is theeffluent withdrawn from the bottom of the stabilization tower of theselective disproportionation unit, line 11 is the benzene product, line12 is the high purity p-xylene product, line 13 is the small amount oftoluene separated from the PX separation unit, line 14 is the mixedxylenes after separating PX, line 15 is the C₈ ⁺A stream withdrawn fromthe bottom of the deheptanizer of the isomerization unit, and line 16 isthe benzene- and toluene-containing stream withdrawn from the top of thedeheptanizer of isomerization unit IV and line 17 is a foreign toluene.

The flow scheme of the traditional complex for producing aromatics isshown in FIG. 1. Benzene and toluene stream 1 from the aromaticsextraction unit and the bottom effluent C₈ ⁺A stream 2 of thedeheptanizer of the catalytic reforming unit enter into the aromaticsfractionation unit I separately and the separated benzene stream 3 andC₁₀ ⁺A stream 7 are withdrawn as the products. Toluene stream 4 and C₉⁺A stream 6 serve as the feed of toluene disproportionation andtransalkylation unit II and mixed xylene stream 5 is fed to xyleneseparation unit III. Whether toluene disproportionation andtransalkylation unit II is equipped with the aromatics separation devicedepends on the concrete situation. When the aromatics separation deviceis equipped, nitration grade benzene product 9 is withdrawn from theselective disproportionation unit and C₈A stream 8 enters into the PXseparation unit to separate PX; when the toluene disproportionation unitis not equipped with the aromatics fractionationunit, bottom effluent 8withdrawn from the stripping tower directly enters into the aromaticsfractionation unit for separation. Target product p-xylene 12 isseparated from p-xylene separation unit III, the small amount of toluene13 returns to the toluene disproportionation unit, and other mixedxylenes 14 enter into xylene isomerization unit IV to conduct theisomerization reaction. C₈ ⁺A stream 15 withdrawn from the bottom ofdeheptanizer of the isomerization unit is fed to the aromaticsfractionation unit and benzene- and toluene-containing stream 16withdrawn from the top of the deheptanizer is fed to the aromaticsextraction unit.

The process flow scheme of the complex for producing aromatics inaccordance with one embodiment of the present invention is shown in FIG.2. Compared to the traditional process, the improvement of the presentinvention is the addition of a toluene selective disproportionation unitbased on the traditional process and the corresponding modification ofthe process route. The similarity between FIG. 1 and FIG. 2 will not bedescribed and the difference will be described bellow in detail. Thepresent process divides the toluene 4 which entirely serves as the feedof the toluene disproportionation and transalkylation unit originallyinto two streams. One still serves as the feed of the toluenedisproportionation transalkylation unit and the remaining toluene servesas the feed of toluene selective disproportionation unit V. Whether theselective disproportionation unit is equipped with the aromaticsseparation device depends on the concrete situation. When the aromaticsseparation device is equipped, nitration grade benzene product 11 iswithdrawn from the selective disproportionation unit. The unreactedtoluene is recycled, and p-xylene-rich C₈A stream 10 enters into thesubzero crystallization separation unit to separate PX; when theselective disproportionation unit is not equipped with the aromaticsfractionation unit, bottom effluent 8 withdrawn from the stripping towerdirectly enters into the aromatics fractionation for separation, and nobenzene stream 11 is withdrawn. Foreign toluene 17 enters the aromaticsfractionation unit I as another stream of the feedstock.

The following examples illustrate the invention without limiting it.

EXAMPLE 1

The capacity for producing PX and energy consumption of the presentinvention was examined based on the composition of various substances ofC₆A-C₁₀ ⁺ aromatics in the aromatics stream withdrawn from the reformingunit of a typical aromatics complex as the fundamental data.

Table 1 is the composition of the aromatics withdrawn from a typicalreforming unit and the flow rates of various components used in thepresent example.

TABLE 1 composition and flow rates of the reformed aromatics ComponentContent, % Flow rate, kg/h Ben 11.65 10000 Tol 43.09 37000 C₈A 31.4527000 C₉A 11.51 9882 C₁₀ ⁺A  2.30 1976 Σ 100 85858

The feedstock shown in Table 1 and the aforesaid technique of thepresent invention (aromatics fractionation device is included in theselective disproportionation unit) are used to produce p-xylene. 75% oftoluene stream 4 enters into the selective disproportionation unit, andthe remaining 25% enters into the disproportionation and transalkylationunit. The weight ratio of toluene to C₉A was 60/40. The catalyst used inthe toluene selective disproportionation unit was a ZSM-5 molecularsieve with a SiO₂/Al₂O₃ mole ratio of 50, on which 0.1% by weight ofplatinum was supported; the reaction conditions were a reaction pressureof 1.5 MPa, a reaction temperature of 420° C., a hydrogen to hydrocarbonmole ratio of 3, and a liquid weight space velocity of 4 h⁻¹. Thecatalyst used in the toluene disproportionation unit was a mordenitemolecular sieve with a SiO₂/Al₂O₃ mole ratio of 30, on which 0.8% byweight of bismuth was supported; the reaction conditions are a reactionpressure of 3.0 MPa, a reaction temperature of 360° C., a hydrogen tohydrocarbon mole ratio of 6, and a liquid weight space velocity of 1.5h⁻¹. Under the above situations, the feed and effluent of the selectivedisproportionation unit, disproportionation and transalkylation unit,and isomerization unit are shown in Table 2, the treating scales ofvarious units of the aromatics complex are shown in Table 3, the outputsof the products p-xylene and benzene are shown in Table 4.

TABLE 2 Process materials in Example 1 Selective DisproportionationAdsorptive disproportionation and transalkylation Isomerizationseparation Component unit unit unit unit Xylene tower Feed Ben 11 3 0 0into Tol 86823 27698 125 125 125 unit, C₈A 84 238 161555 21447 214661Kg/h C₉A / 18908 41 41 18979 C₁₀ ⁺A / 657 3632 Σ 86918 47505 161722214613 237398 Flow- Ben 12006 3866 2061 0 0 out of Tol 59799 16818 1043125 125 unit, C₈A 12195 15704 155915 214447 214661 kg/h C₉A 703 7540 82541 18979 C₁₀ ⁺A 20 1458 3632 Σ 84723 45386 159844 214613 237398 WhereBen is benzene, Tol is toluene, C₈A is C₈ aromatics, C₉A is C₉aromatics, and C₁₀ ⁺A is C₁₀ and higher aromatics.

TABLE 3 Treating scales of various units in Example 1 DisproportionationAdsorptive Xylene and transalkylation Isomerization separationBenzene/toluene fractionation Unit unit unit unit fractionation unitunit Scale, kg/h 47505 161722 210658 96240 233214

TABLE 4 Outputs and purities of the products in Example 1 ProductOutput, kg/h Purity, % p-xylene 48936 99.8 Benzene 27933 99.94

The results show that the energy consumption was 575 Mkcal/t(p-xylene+benzene), which was reduced by 18% in the technique of thepresent invention relative to the 702 Mkcal/t (p-xylene+benzene) in thefollowing Comparative Example 1. In the present invention, the weightconcentration of the p-xylene in the mixed xylenes entering into theadsorptive separation unit was 23.0%, which was 20% higher than that inthe Comparative Example 1 (19.2%).

EXAMPLE 2

By still using the process flow, process conditions, and catalyst inExample 1, the capacity of the technique of the present invention forproducing PX was re-examined based on the composition of the feedstockand 22% more flow rate of the feed shown in Table 1. The composition andflow rate of the feed are shown in Table 5. The specific situations ofthe feed and effluent of the selective disproportionation unit,disproportionation and transalkylation unit, and isomerization unit areshown in Table 6, the treating scales of various units of the aromaticscomplex are shown in Table 7, and the outputs of the products p-xyleneand benzene are shown in Table 8.

TABLE 5 Contents and flow rates of the reformed aromatics ComponentContent, % Flow rate, kg/h Ben 11.65 12200 Tol 43.09 45140 C₈A 31.4532940 C₉A 11.51 12056 C₁₀ ⁺HCs 2.30 2410 Σ 100 104746

TABLE 6 Process materials in Example 2 Selective DisproportionationAdsorptive disproportionation and transalkylation Isomerizationseparation Component unit unit unit unit Xylene tower Feed Ben 13 4 0 0into Tol 105875 33847 153 125 125 unit, C₈A 102 295 201195 214447 214661Kg/h C₉A / 23106 50 41 18979 C₁₀ ⁺A / 3819 3632 Σ 105991 61071 201397214613 237398 Flow- Ben 14641 5323 2566 0 0 out of Tol 72921 20535 1296125 125 unit, C₈A 14871 20126 194172 214447 214661 kg/h C₉A 857 92171026 41 18979 C₁₀ ⁺A 24 2017 3632 Σ 103314 57218 199060 214613 237398

TABLE 7 Treating scales of various units in Example 2 DisproportionationAdsorptive Xylene and transalkylation Isomerization separationBenzene/toluene fractionation Unit unit unit unit fractionation unitunit Scale, kg/h 61071 201397 261926 118380 289732

TABLE 8 Outputs and purities of the products in Example 2 ProductOutput, kg/h Purity, % p-xylene 60528 99.8 Benzene 34731 99.94

Compared to Comparative Example 1, the treating capacity of the devicein Example 2 increased by 22%, the output of p-xylene increased by18.4%, and the output of benzene increased by 30.8%, while the treatingscales of the other units did not increase except the selectivedisproportionation unit, which was not equipped in ComparativeExample 1. The nearest treating capacities were in the adsorptiveseparation unit, but the treating capacity was only 99.8% of that inComparative Example 1. Therefore, by using the technique of the presentinvention, the treating capacity of the device and the output of theproducts p-xylene and benzene can be increased.

EXAMPLE 3

By still using the process flow, process conditions, and catalyst inExample 1, the capacity of the technique of the present invention forproducing PX was re-examined based on the flow rate of the feedstockshown in Table 1, but purchased 53000 kg/h pure toluene entered into theselective disproportionation unit. The specific situations of the feedand effluent of the selective disproportionation unit,disproportionation and transalkylation unit, and isomerization unit areshown in Table 9, the treating scales of various units of the aromaticscomplex are shown in Table 10, and the outputs of the products p-xyleneand benzene are shown in Table 11.

TABLE 9 Process materials in Example 3 Selective DisproportionationAdsorptive disproportionation and transalkylation Isomerizationseparation Component unit unit unit unit Xylene tower Feed Ben 22 3 0 0into Tol 256328 31722 245 125 125 unit, C₈A 201 288 183099 214447 214661Kg/h C₉A / 21647 41 41 18979 C₁₀ ⁺A / 688 3632 Σ 256551 54349 183386214613 237398 Flow- Ben 34969 4392 2277 0 0 out of Tol 177790 19261 1270125 125 unit, C₈A 35437 17858 176776 214447 214661 kg/h C₉A 2046 8839922 41 18979 C₁₀ ⁺A 57 1578 3632 Σ 250299 51928 181245 214613 237398

TABLE 10 Treating scales of various units in Example 3Disproportionation Adsorptive Xylene and transalkylation Isomerizationseparation Benzene/toluene fractionation Unit unit unit unitfractionation unit unit Scale, kg/h 54349 183386 256880 103347 82395

TABLE 11 Outputs and purities of the products in Example 3 ProductOutput, kg/h Purity, % p-xylene 73494 99.8 Benzene 51638 99.94

The results show that in the present example, the output of p-xyleneincreases by 45%, and the output of benzene increases by 197%, while thetreating scales of the other units did not increase except the selectivedisproportionation unit, which was not equipped in ComparativeExample 1. Therefore, by using the technique of the present invention,the treating capacity of the system and the outputs of the productsp-xylene and benzene can be greatly increased by only setting up a newselective disproportionation unit containing an aromatics fractionationunit and purchasing an adequate amount of toluene, but keeping the scaleof the original units unvaried. In addition, the weight concentration ofthe p-xylene in the mixed xylenes entering into the adsorptiveseparation unit increases by 29.6%.

EXAMPLE 4

By still using the process flow, process conditions, and catalyst inExample 1, and the same feedstock as Example 3, the capacity of thetechnique of the present invention for producing PX was re-examined, butthe difference was that the process for separating the mixed xylenesproduced by the toluene selective disproportionation changed from theadsorptive separation to the subzero crystallization separation. Thetreating scales of various units of the aromatics complex are shown inTable 12, and the outputs of the products p-xylene and benzene are shownin Table 13.

TABLE 12 Treating scales of various units in Example 4Disproportionation Adsorptive Xylene Subzero- and transalkylationIsomerization separation Benzene/toluene fractionation crystallizationUnit unit unit unit fractionation unit unit unit Scale, kg/h 54323182414 232553 103315 258054 37573

Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to a preferred embodimentthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the processes illustrated, and intheir operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention. For example, it is expresslyintended that all combinations of those elements and/or method stepswhich perform substantially the same function in substantially the sameway to achieve the same results are within the scope of the invention.Moreover, it should be recognized that elements and/or method stepsshown and/or described in connection with any disclosed form orembodiment of the invention may be incorporated in any other disclosedor described or suggested form or embodiment as a general matter ofdesign choice. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto.

We claim:
 1. A process or the selective disproportionation of tolueneand the disproportionation and transalkylation of toluene and C₉ ⁺aromatics comprising: a) separating a feed stream comprising benzene,toluene, C₈ aromatics, and C₉ ⁺ aromatics into a first benzene stream, atoluene stream, a first C₈ aromatics stream, and a C₉ ⁺ aromaticsstream; b) introducing part of the toluene stream into a tolueneselective disproportionation unit to conduct the toluene selectivedisproportionation reaction for producing a first effluent comprising C₈aromatics rich in p-xylene, and benzene; c) separating a second C₈aromatics stream and a second benzene stream from the first effluentproduced in the step of b); d) introducing another part of the toluenestream and the C₉ ⁺ aromatics stream into a toluene disproportinationand transalkylation unit to conduct the toluene disproportionation andtransalkylation reaction in the presence of hydrogen for producing asecond effluent comprising C₈ aromatics and bezene; e) separating athird C₈ aromatics stream and a third benzene stream from the secondeffluent produced in the step of d); and f) separating the first, andthird C₈ aromatics stream into a p-xylene product and a remaining mixedxylenes stream.
 2. The process of claim 1 further comprising a step ofseparating the second C₈ aromatics into the p-xylene product and theremaining mixed xylenes stream.
 3. The process of claim 1 wherein theamount of the toluene introduced in the step of b) accounts for 5-95% ofthe total amount of the toluene introduced in the steps of b) and d). 4.The process of claim 1 wherein the amount of the toluene introduced inthe step of b) accounts for 20-80% of the total amount of the tolueneintroduced in the steps of b) and d).
 5. The process of claim 1 whereinthe weight ratio of the toluene and the C₉ ⁺ aromatics introduced in thestep of d) is 80/20-40/60.
 6. The process of claim 3 wherein the totalamount of the toluene includes the toluene separated in the step of a)and a foreign toluene.
 7. The process of claim 1 further comprisingsteps of sending the remaining mixed xylenes stream to an isomerizationunit to conduct the reaction of isomerization for converting theremaining mixed xylenes to the p-xylene; separating the p-xylene productfrom the effluent of the reaction of isomerization.
 8. The process ofclaim 1 wherein a C₁₀ ⁺ aromatics stream is also separated from the feedstream in the step of a) when the C₁₀ ⁺ aromatics is present in the feedstream.
 9. The process of claim 1 wherein the p-xylene is separated byutilizing an adsorptive separation method.
 10. The process of claim 1wherein the p-xylene is separated by utilizing a subzero crystallizationseparation method.
 11. The process of claim 1 wherein the step of c) isconducted independently in an aromatics fractionation unit specificallyequipped for the step of c), and the second C₈ aromatics stream isfurther separated into a part of the p-xylene product and a residualliquid by the subzero crystallization separation method.
 12. The processof claim 11 further comprising steps of: admixing the residual liquidwith the first and the third C₈ aromatics stream to form a mixture; andseparating the mixture into the p-xylene product and the remaining mixedxylene by the adsorptive separation method.
 13. The process of claim 1wherein the reaction zone of the toluene selective disproportionationunit is normally operated at a reaction pressure of 1-4 MPa, a reactiontemperature of 300-480° C., a hydrogen to hydrocarbon mole ratio of0.5-10, and a liquid weight space velocity of 0.8-8 h^(−1.)
 14. Theprocess of claim 1 wherein the reaction zone of the disproportionationand transalkylation unit is normally operated at a reaction pressure of1-5 MPa, a reaction temperature of 250-480° C., a hydrogen tohydrocarbon mole ratio of 0.5-10, a liquid weight space velocity of0.8-8 h⁻¹, and a content of the C₉ ⁺ aromatics of 5-60% by weight in thetoluene stream an the C₉ ⁺ aromatics stream introduced in the step ofd).
 15. The process of claim 1 wherein the toluene selectivedisproportionation unit is normally operated in the presence of aZSM-type molecular sieve catalyst.
 16. The process of claim 15 whereinthe ZSM-type molecular sieve catalyst is a ZSM-5-type molecular sievewith a SiO₂/Al₂O₃ mole ratio of 15-100.
 17. The process of claim 16wherein the ZSM-type catalyst contains a 0.01-1% by weight of metal oroxide thereof, the metal is selected from the group consisting ofplatinum, molybdenum, magnesium, and combinations thereof.
 18. Theprocess of claim 1 wherein the disproportionation and transalkylationunit is normally operated in the presence of a molecular sieve typecatalyst selected from the group consisting of β-zeolite, mordenite,MCM-22 and combinations thereof.
 19. The process of claim 18 wherein themolecular sieve type catalyst is selected from the group consisting ofβ-zeolite, mordenite, and combinations thereof, the molecular sieve hasa SiO₂/Al₂O₃ mole ratio of 15-50.
 20. The process of claim 18 whereinthe molecular sieve type catalyst contains 0.05-2% by weight of metalbismuth or its oxide.